Patent Literature 1 (Japanese Laid-Open Patent Publication No. 2013-168865) discloses an on-vehicle network system as follows. That is, the on-vehicle network system includes: an on-vehicle control unit having a memory that stores therein definition data defining a part, of protocol used in an on-vehicle network, which depends on implementation on the on-vehicle network; and a protocol issuing device that issues the definition data to the on-vehicle control unit. When the protocol issuing device receives, from a registration device that allows the on-vehicle control unit to participate in the on-vehicle network, a registration request that requests participation of the on-vehicle control unit in the on-vehicle network, the protocol issuing device performs authentication for the registration device, creates the definition data based on implementation on the on-vehicle network, and returns the definition data to the registration device. The registration device receives the definition data transmitted from the protocol issuing device, and requests the on-vehicle control unit to store the received definition data in the memory. Then, the on-vehicle control unit receives the definition data from the registration device, stores the definition data in the memory, and performs communication by using the on-vehicle network, based on the part, of the protocol, defined by the definition data.